This invention relates to a rare earth metal-nickel hydrogen occlusive alloy ingot having crystalline structures excellent in hydrogen storage and a method for producing same.
Up to now, a hydrogen occlusive alloy ingot is prepared by a metal mold casting method of casting a molten alloy into a metal mold. On the other hand, a variety of additive elements such as cobalt, manganese or aluminum are usually added to the alloy for improving its characteristics such as hydrogen occluding characteristics. However, if the molten alloy is to be solidified by the metal mold casting method, the heat conduction through the casting mold determines the rate of heat removal during an initial stage of a heat removal process of the molten alloy. As solidification proceeds, the heat conduction between the casting mold and the solidifying phase or in the solidifying phase determines the rate of heat conduction. Even though the cooling capacity of the metal mold is increased, the inner portions of the ingot and those portions thereof in the vicinity of the casting mold are subjected to different cooling conditions. Such phenomenon is the more pronounced, the thicker the ingot thickness. The result is that in the case of a larger difference between the cooling conditions in the inner portions of the ingot and those in the vicinity of the ingot surface, microscopic segregation of additive elements, above all, is likely to occur. Besides, a homogenizing heat treatment process needs to be carried out fop a prolonged period of time for homogenizing the crystal composition.
On the other hand, the ingot is usually pulverized to a size of an order of tens of microns during the pulverization step in the course of the process for preparation of hydrogen occlusive alloy powders. However, the ingot produced by the metal mold casting method contains a phase rich in additive elements having a larger particle size and hence presenting difficulties in pulverization. Consequently, the particle size of the powders from the pulverization process becomes non-uniform to adversely affect hydrogen occlusive characteristics, thus rendering it impossible to obtain unobjectionable hydrogen occlusive characteristics of the ultimate hydrogen occlusive alloy powders.